The thermal properties of photonic crystal fiber (PCF) laser with 18 circularly distributed cores are investigated by using full-vector finite element method (FEM). The results show that the 18-core PCF has a more effective thermal dispersion construction compared with the single core PCF and 19-core PCF. In addition, the temperature distribution of 18-core PCF laser with different thermal loads is simulated. The results show that the core temperature approaches the fiber drawing value of 1800 K approximately when the thermal load is above 80 W/m which corresponds to the pumping power of 600 W approximately, while the coating temperature approaches the damage value of about 550 K when the thermal load is above 15 W/m which corresponds to the pumping power of 110 W approximately. Therefore the fiber cooling is necessary to achieve power scaling. Compared with other different cooling systems, the copper cooling scheme is found to be an effective method to reduce the thermal effects.